1. Field of the Invention
The invention relates to an audio transmission circuit comprising a telephone amplifier, for an electronic telephone set having line terminals for connecting the set to a subscriber line, this set being fed with direct current via the subscriber line and further comprising, arranged between the line terminals, a series arrangement of a line terminating impedance determining the alternating current impedance of the set, and a supply capacitor for supplying the audio transmission circuit, this audio transmission circuit having a supply input for connection to the junction between the line terminating impedance and the supply capacitor.
2. Background of the Invention
Such an audio transmission circuit is disclosed in the non-prepublished United Kingdom patent application No. 8,224,347.
In addition to a transmission circuit for audio signals, electronic telephone sets often comprise further electronic circuits such as, for example, a circuit for generating tones for tone-push button dialling, a circuit for automatically repeated dialling, memory circuits or the control circuits associated with a display.
With telephone sets whose direct current supply is effected via the subscriber line, the total current available for supplying this circuit is limited. The available supply current is determined inter alia by the resistance, and thus by the length of the subscriber line. More specifically with the maximum subscriber line length allowed by the administration, it is possible that insufficient direct current is available to feed all the electronic circuits of the telephone set simultaneously.